Clipping SVG output

In a message to the qt-interest mailing list, Jeremy Sanders asked if it was possible to clip polylines before sending them to a paint device. This was partly to overcome the limitations of Qt's SVG paint device, QSvgGenerator, which doesn't support clipping.

svg_output.png

The following example code shows how to clip painter output when generating an SVG, using a QPainterPath as a stencil by passing the painter paths to be rendered to its QPainterPath.intersected() method and drawing the result. Note that this only works for pens with zero width.

 1 import random, sys
 2 from PyQt4.QtCore import QSize, Qt
 3 from PyQt4.QtGui import *
 4 from PyQt4.QtSvg import *
 5 
 6 def randomColor():
 7 
 8  red = 205 + random.random() * 50
 9  green = 205 + random.random() * 50
 10  blue = 205 + random.random() * 50
 11  alpha = 91 + random.random() * 100
 12 
 13  return QColor(red, green, blue, alpha)
 14 
 15 
 16 if __name__ == "__main__":
 17 
 18  if len(sys.argv) != 2:
 19  
 20  sys.stderr.write("Usage: %s <output path>\n" % sys.argv[0])
 21  sys.exit(1)
 22  
 23  app = QApplication(sys.argv)
 24  
 25  svg = QSvgGenerator()
 26  svg.setFileName(sys.argv[1])
 27  svg.setResolution(300)
 28  svg.setSize(QSize(1024, 768))
 29  
 30  random.seed()
 31  
 32  p = QPainter()
 33  p.begin(svg)
 34  
 35  clipPath = QPainterPath()
 36  clipPath.addRect(0, 0, 1024, 768)
 37  
 38  gradient = QLinearGradient(0, 0, 1024, 768)
 39  gradient.setColorAt(0, QColor(0, 64, 64))
 40  gradient.setColorAt(1, QColor(0, 0, 64))
 41  p.fillRect(0, 0, 1024, 768, QBrush(gradient))
 42  p.setPen(Qt.NoPen)
 43  
 44  font = QFont("FreeSans")
 45  
 46  for i in range(100):
 47  
 48  w = 10 + random.random() * 200.0
 49  h = 0.5 * (1 + random.random()) * w
 50  color = randomColor()
 51  x = random.random() * 1024
 52  y = random.random() * 768
 53  
 54  rectPath = QPainterPath()
 55  rectPath.addRoundedRect(x - w/2, y - h/2, w, h, 20, 20)
 56  
 57  p.setBrush(color)
 58  p.drawPath(clipPath.intersected(rectPath))
 59  
 60  font.setPixelSize(min(w/2, h/2))
 61  metrics = QFontMetrics(font, svg)
 62  text = chr(random.randrange(97, 123))
 63  rect = metrics.boundingRect(x - w/2, y - h/2, w, h, Qt.AlignCenter, text)
 64  textPath = QPainterPath()
 65  textPath.addText(x - rect.width()/2, y + rect.height()/2 - metrics.descent(), font, text)
 66  
 67  p.setBrush(QColor(0, 0, 0, color.alpha()))
 68  p.drawPath(clipPath.intersected(textPath))
 69  
 70  p.end()
 71  
 72  sys.exit()

One way to deal with non-zero-width pens is to use QPainterPathStroker. This creates an outline of a painter path, converting pen strokes to fillable paths. We can use this to create paths which we can then effectively clip using the stencil path.

svg_output_stroked.png

 1 import random, sys
 2 from PyQt4.QtCore import QSize, Qt
 3 from PyQt4.QtGui import *
 4 from PyQt4.QtSvg import *
 5 
 6 def randomColor():
 7 
 8  red = 205 + random.random() * 50
 9  green = 205 + random.random() * 50
 10  blue = 205 + random.random() * 50
 11  alpha = 91 + random.random() * 100
 12  
 13  return QColor(red, green, blue, alpha)
 14 
 15 
 16 if __name__ == "__main__":
 17 
 18  if len(sys.argv) != 2:
 19  
 20  sys.stderr.write("Usage: %s <output path>\n" % sys.argv[0])
 21  sys.exit(1)
 22  
 23  app = QApplication(sys.argv)
 24  
 25  svg = QSvgGenerator()
 26  svg.setFileName(sys.argv[1])
 27  svg.setResolution(300)
 28  svg.setSize(QSize(1024, 768))
 29  
 30  random.seed()
 31  
 32  p = QPainter()
 33  p.begin(svg)
 34  
 35  clipPath = QPainterPath()
 36  clipPath.addRect(0, 0, 1024, 768)
 37  
 38  stroker = QPainterPathStroker()
 39  stroker.setWidth(16)
 40  stroker.setDashPattern([0.1, 1.5])
 41  
 42  gradient = QLinearGradient(0, 0, 1024, 768)
 43  gradient.setColorAt(0, QColor(0, 64, 64))
 44  gradient.setColorAt(1, QColor(0, 0, 64))
 45  p.fillRect(0, 0, 1024, 768, QBrush(gradient))
 46  p.setPen(Qt.NoPen)
 47  
 48  font = QFont("FreeSans")
 49  strokeColor = QColor(0, 127, 255)
 50  
 51  for i in range(100):
 52  
 53  w = 10 + random.random() * 200.0
 54  h = 0.5 * (1 + random.random()) * w
 55  color = randomColor()
 56  x = random.random() * 1024
 57  y = random.random() * 768
 58  
 59  rectPath = QPainterPath()
 60  rectPath.addRoundedRect(x - w/2, y - h/2, w, h, 20, 20)
 61  
 62  strokedPath = stroker.createStroke(rectPath)
 63  
 64  # Draw the background of the rounded rectangle.
 65  p.setBrush(color)
 66  p.drawPath(clipPath.intersected(rectPath))
 67  # Draw the outline of the rounded rectangle.
 68  p.setBrush(strokeColor)
 69  p.drawPath(clipPath.intersected(strokedPath))
 70  
 71  font.setPixelSize(min(w/2, h/2))
 72  metrics = QFontMetrics(font, svg)
 73  text = chr(random.randrange(97, 123))
 74  rect = metrics.boundingRect(x - w/2, y - h/2, w, h, Qt.AlignCenter, text)
 75  textPath = QPainterPath()
 76  textPath.addText(x - rect.width()/2, y + rect.height()/2 - metrics.descent(), font, text)
 77  
 78  p.setBrush(QColor(0, 0, 0, color.alpha()))
 79  p.drawPath(clipPath.intersected(textPath))
 80  
 81  p.end()
 82  
 83  sys.exit()